After drilling a well for hydrocarbons, it may be necessary to perforate the walls of the well to facilitate flow of hydrocarbons into the well. Wells require perforation because the drilling process causes damage to the formation immediately adjacent to the well. This damage reduces or eliminates the pores through which the oil or gas would otherwise flow. Perforating the well creates a channel through the damage to undamaged portions of the formation. The hydrocarbons flow through the formation pores into the perforation channels and through the perforation channels into the well itself.
In addition, steel casing may be set within the hole adjacent to the hydrocarbon bearing formation. The casing forms a barrier that prevents flow of the hydrocarbons into the well. In such situations, the perforations go through the casing before entering the formation.
Traditional methods of perforating the well (both casing and the formation) involved lowering tools that contain explosive materials into the well adjacent to the hydrocarbon bearing formation. Discharge of the explosive would either propel a projectile through the casing and into the formation or, in the case of shaped charges, directly create a channel with explosive force. Such devices and methods are well known in the art.
In vertical wells, gravity may be used to lower the perforating device into position with wireline being used to hold the device against gravity and retrieve the device after discharge. For lateral wells, which may be horizontal or nearly horizontal, gravity may only be used to lower the perforating device to a point where the friction of the device against the well bore overcomes the gravitational force. The perforating device must then be either pushed or pulled along the lateral portion of the well until the device reaches the desired location.
For wireline conveyed devices, motorized devices called tractors, which are well known in the art, are sometime used to pull the explosive perforating device into position. Tractors, however, can be unreliable and may be damaged by the explosive force of the perforating device.
Another method for positioning the perforating device is with coiled tubing. This technique is sometimes called tubing conveyed perforation or TCP. One advantage of TCP is that the perforating device is attached to the end of the coiled tubing and the coiled tubing pushes the device into the proper location. For lateral wells, the tubing will often contain wireline within the coiled tubing. The wireline can be used to carry an electric current to discharge the explosive contained within the perforating device.
Another advantage of tubing conveyed perforation is the ability to set a hydraulic bridge plug at a location in the well below (distal in relation to the wellhead) the relevant hydrocarbon bearing formation, or between two hydrocarbon bearing formations. This allows the producing zones of the well to be isolated. Once the bridge plug is set, the perforating device can be fired and any fluids from the newly perforated zone will not flow into any regions separated by the bridge plug.
Special explosive perforating devices have been developed that contain a channel for the flow of hydraulic fluid. Thus, the bridge plug can be set, and the perforating device discharged with a single trip of the coiled tubing. Without a flow channel in the perforating device, the tubing end would have to return to the surface, have a perforating device attached, and return to the hydrocarbon bearing formation before perforation can be performed. Thus, the ability to set the bridge plug and perforate in a single trip saves significant time.
While the perforating devices used in prior art methods of TCP have provided the ability to set a bridge plug and perforate the well in a single trip, the methods are still limited. For example, the length of the perforated zone is limited to the length of perforating gun assembly. In other words, to perforate along a 100 foot length of the well, the perforating gun assembly must be at least 100 feet long. This does not include the length of the bridge plug at the end of the gun assembly. However, the increased length also increases the mass of the gun assembly, making the assembly more difficult to deploy in horizontal wells.
Long gun assemblies have an additional disadvantage. The gun assembly is introduced into the well using a lubricator. The lubricator is a device attached to the well head below the coiled tubing or wireline injector, depending on whether tubing or wireline is used to convey the gun assembly. The length of the lubricator is directly related to the length of the gun assembly. If the gun assembly is 100 feet long, the lubricator is at least the same length. In such a case, the injector, either coiled tubing or wireline, above the lubricator is at least 100 feet in the air which creates difficulties running hydraulic hoses, control lines, and with maintenance should the injector head fail.
One alternative to the explosive perforating device is an abrasive perforating device. Abrasive perforating devices direct a concentrated stream of fluid against the casing and, once the casing is penetrated, the surrounding formation. The fluid contains a suspended solid or solids, such as sand, to wear away the metal and rock of the casing and formation. Abrasive perforation is well known in the art.
The operator merely increases flow of the abrasive fluid to begin perforation and decreases flow to stop perforation. The depth and size of perforations are controlled by the fluid pressure and by the length of perforation time. With an abrasive perforator, perforations can be made across a long interval of the well in a single trip and without increasing the size of the tool string. Thus abrasive perforators avoid the problems created by the increased size and weight of long gun assemblies.
Prior art abrasive perforation devices have been run on the end of tool strings. Thus, the fluid channel ends at the bottom of the abrasive perforating device. This configuration has prevented the addition of other tools, such as bridge plugs, below the abrasive perforating device. As mentioned above, running a bridge plug or other tool below the abrasive perforator is sometimes desirable.